The present invention relates to a developing device for use in an image forming apparatus and of the type having a plurality of developing sleeves arranged one above the other in a predetermined relationship along the surface of a latent image carrier which is included in the image forming apparatus. This type of developing device develops a latent image electrostatically formed on the image carrier by using a toner contained in a developer which is fed to the developing sleeves.
An electrophotographic copier, facsimile machine, laser printer or similar image forming apparatus electrostatically forms a latent image on a photoconductive element or similar image carrier and develops the latent image to render it visible. This kind of apparatus has a developing device which is often implemented with a two-component powdery developer, i.e., a mixture of toner and carrier. The developing device usually has a single developing sleeve located to face the photoconductive element, and a paddle wheel serving as a means for supplying the developer to the photoconductive element. The photoconductive element and the developing sleeve rotate in the same direction in a developing region where they face each other. Assuming that the linear velocities of the photoconductive element and developing sleeve are Vp and Vs, respectively, the ratio Vs/Vp is ordinarily selected to be 3 or above. Specifically, the linear speed of the developing sleeve is selected to be far higher than that of the photoconductive element, so that an amount of developer great enough to produce a toner image having a predetermined density may be fed to the developing region. Such a relationship between the photoconductive element and the developing sleeve is disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 58-207064, for example. This, however, brings about a problem that providing a vertical and a horizontal thin line image extending respectively in the rotating direction and the axial direction of the photoconductive element with a predetermined width is difficult. Another problem is that when a cruciform image is formed on the photoconductive element, trailing end portions of the image with respect to the rotating direction of the photoconductive element, i.e., the moving direction of its surface are often lost and left blank on a paper sheet.
Another type of developing device extensively used today has a plurality of developing sleeves, such as two developing sleeves, arranged one above the other along the surface of the photoconductive element. In this type of developing device, a paddle wheel supplies a developer to the developing sleeves, while a separator and a doctor regulates the amount of developer being supplied by the paddle wheel. Assume that the two developing sleeves are rotated at the same linear velocity of Vs, and the photoconductive element is rotated at a linear velocity Vp. Then, even if the ratio Vs/Vs is as small as 1.5 or so, a sufficient amount of developer can be fed to the developing regions where the individual developing sleeves are located. This is successful in uniformizing the widths of vertically extending and horizontally extending thin line images on the photoconductive element and in eliminating the local omission of a cruciform image as mentioned previously. However, such a multiple sleeve scheme gives rise to another problem, as follows.
The upper and lower developing sleeves are spaced apart from each other by a gap of about 1 millimeter. The gap causes a part of the developer fed by the paddle wheel to be directly transported to the photoconductive element therethrough. Consequently, the amount of developer to be fed to the upper sleeve by the separator and doctor is reduced and, hence, the amount of developer passing the doctor becomes unstable. The developer conveyed toward the photoconductive element through the gap is fed only to the developing region where the lower sleeve is located and not to the developing region where the upper sleeve is located. When a substantial amount of developer flows through the gap, it will accumulate at the upstream side of the developing region where the lower sleeve is disposed and exert, in due course, a substantial load on the photoconductive element and developing sleeves. Such a load is apt to prevent the photoconductive element and the developing sleeves from being rotated at their predetermined speeds. The developer flowing through the gap and not through the separator and doctor is not stressed and, therefore, fails to have its toner and carrier sufficiently charged. It is, therefore, likely that the toner particles are released from the carrier and deposited on the photoconductive element, contaminating the background area on the photoconductive element. Further, such floating toner particles are apt to smear the interior of the image forming apparatus. In any case, the developer flowing through the gap obstructs stable development. To eliminate this problem, a developer supply sleeve having magnets thereinside may be located above the paddle wheel and in close proximity to the upper developing sleeve. In this case, the developer will be fed by the paddle wheel to the upper developing sleeve by way of the developer supply sleeve and not through the gap. However, the developer supply sleeve accommodating magnets thereinside would increase the overall dimensions of the developing device and thereby the production cost thereof.